The investigation aimed to gain an understanding of the medieval phases of the monument’s use. A number of rooms in the Ist order (ground floor) were examined, as were a space below the stairs of cuneo III on the third level and all of cuneo X (space below the stairs, in front of the radial corridor and part of the third level). The present floor level, created by 17th century restructuring, from which the excavation began, coincides approximately with the ancient one. The excavation investigated the layers which had formed, following the almost total robbing of the ancient flooring, between the opus caementicium of the Flavian foundations and the level fixed after the in-fills of the 19th century, which eliminated all modern and part of the late medieval stratification. In cuneo III the layer filling the robbing of the ancient paving, although cut and disturbed by later interventions, fixed the chronology of the robbing to the second half of the 12th century, in agreement with what has been documented in the innermost part of cuneo X, the ambulatory and radial corridor.

The situation was more complex in the area below the stairs in cuneo X. Here, the excavation exposed various occupation phases following the removal of the paving. During the 12th century, soon after the robbing of the paving slabs and almost at the level of the Flavian foundations, the opening onto the ambulatory was closed by a wall (of which only a slight trace remains) and an uneven beaten earth floor was put down. On the rear side, abutting the ancient wall which closed it, a structure was built with stones of varying sizes and lime mortar, perhaps a counter, of which only the lowest part is preserved. A hearth constituted by a circle of stones, suggests that this space may have been used as a dwelling. This evidence shows that types of temporary dwelling, for which both archaeological and documentary evidence is very scarce, still existed in the late medieval period.

Subsequently, during the first half of the 13th century, the space was radically modified. The floor level was raised by up to 30 cm by a dump of earth and stones, and a new uneven floor surface was created. The structure up against the back wall, and the wall which had closed it off from the ambulatory were demolished. A new structure was built up against the east wall, constituted by a wall about 1.20 m high and about 5 m long, abutted by a tank made up of a thick layer of opus signinum. It is likely that this transformation marked a change in use for this space, it may have become a storeroom, animal stall or been used for production activities that have yet to be defined. In the 13th century it fell out of use, as attested by the robber trenches of the Flavian stone block walls which had delimited the space. It was covered by silt deposits, cut by 17th century digging. The upper part of the stratification was constituted by the layers relating to the reconstruction undertaken in the first half of the 19th century.